Elastomeric homopolymers and copolymers of more than one monomer are well known in the art. A particularly useful class of synthetic elastomers comprises adjacent blocks of a polymerized elastomer such as a conjugated alkadiene and a polymerized non-elastomer such as styrene wherein the blocks have narrow molecular weight distributions. Anionic polymerization of such block copolymers is well known in the art and a number are commercial, being marketed by Shell Chemical Company as KRATON.RTM. Thermoplastic Rubber.
Cationic polymerization of elastomeric polymers is described in U.S. Pat. No. 4,946,899. A cationic polymerization initiator of desired functionality is reacted with one or more monomers in the presence of a complex of a Lewis acid such as titanium tetrachloride or boron trichloride and an electron pair donor (Lewis base) such as ethyl acetate, dimethyl acetamide, dimethyl formamide, dimethyl sulfoxide, hexamethyl phosphoric triamide, N-methyl-2-pyrrolidinone, pyridine, acetone, methylethyl ketone, etc. Elastomeric polymers of isobutylene, including block copolymers with styrene or styrene derivatives, were found to have narrower molecular weight distributions than previously achieved by cationic processes. However, the molecular weight distributions are significantly broader than is achieved b anionic polymerization.
Further narrowing of the molecular weight distributions achieved by cationic polymerization is essential to prepare elastomeric polymers having comparable properties to polymers produced by anionic polymerization.